This invention relates to polyacetal carboxylate compositions that are useful as detergent builders, and more particularly to the preparation of particulate polyacetal carboxylate by agglomeration.
Polyacetal carboxylate, or polymeric acetal carboxylate, ordinarily comprises a mixture of polymeric acetal carboxylates of various chain lengths. Thus, polyacetal carboxylate useful as a detergent builder generally corresponds to the formula ##STR2## where n is at least 4, preferably between about 10 and about 400, and represents the average chain length of the polyacetal carboxylate in the mixture, R.sub.1 and R.sub.2 are stabilizing end groups, and M is an alkali metal, ammonium, an alkyl group of 1 to about 4 carbon atoms, a tetraalkyl ammonium group, or an alkanolamine group having from 1 to about 4 carbon atoms in the alkyl chain. Polyacetal carboxylate is incorporated in dry detergent formulations as a builder for chelation or sequestration of metal ions ordinarily found in tap water. Such polyacetal carboxylate, or "polymeric acetal carboxylate", is described in more detail in U.S. Pat. No. 4,144,226 to Crutchfield et al.
By chelating or sequestering metal ions such as calcium, magnesium, zinc, lead, copper and cadmium ions, the builder softens the water, thereby improving the effectiveness of the detergent. Such detergent formulations are adapted for employment by consumers for a number of end uses, including automatic dishwashing and laundering of clothing. In addition to a surfactant and builder, detergent formulations typically contain other ingredients such as alkalis, dyes, perfumes and the like. Ordinarily, a solid, particulate mixture of polyacetal carboxylate, sodium carbonate and water, generally in the form of water of hydration, is added to the detergent. A typical polyacetal carboxylate mixture contains between about 76% by weight and about 82% by weight polyacetal carboxylate such as sodium polyglyoxylate, between about 0.2% by weight and about 1% by weight monomeric glyoxylate, between about 6% by weight and about 12% by weight water and other volatiles, between about 2% by weight and about 5% by weight sodium carbonate, between about 0.5% by weight and about 1.5% by weight oxalate salt, and trace amounts of sodium glycolate and sodium formate.
In commercial detergent formulation practice, a builder such as polyacetal carboxylate may be incorporated by dry mixing of particulate polyacetal carboxylate, or polyacetal carboxylate mixture, with particulate detergent, but this process does not always provide a product in which the polyacetal carboxylate is uniformly distributed, or in which initial uniformity of distribution is maintained during storage and shipment. A uniform polyacetal carboxylate/detergent mixture is desirable so that the consumer obtains a proper proportion of builder to detergent each time a portion of the mixture is scooped out of, or poured from, a box of the detergent mixture. However, when produced by conventional precipitation processes, particulate polyacetal carboxylate includes a large proportion of very small, dust-like particles, and sometimes also includes relatively over-sized particles, which results in segregation of the polyacetal carboxylate from the detergent in the box. Polyacetal carboxylate particles that are substantially smaller than the particles of detergent tend to fall to the bottom of the detergent box. If any polyacetal carboxylate particles are substantially larger and less dense than the detergent particles, such particles tend to rise to the top of the box. Therefore, there is a need for a relatively inexpensive method for preparation of polyacetal carboxylate in the form of particles of substantially uniform size, on the order of the size of the detergent granules with which the polyacetal carboxylate is to be mixed.
Conventionally, polyacetal carboxylate is prepared from a slurry as produced, for example, by the saponification process disclosed in U.S. Pat. No. 4,140,676 to Crutchfield. The polyacetal carboxylate is prepared from the slurry by precipitation with methanol. The preciptate is centrifuged, and then washed and dried. If a dynamic drying process is used, the dry centrifuge cake is generally of such small particle size that it is added directly to the dry detergent formulation. But if a static drying process is employed, the cake must be milled before addition to the dry detergent formulation. These conventional procedures, however, involve several drawbacks. As noted, these methods for preparing particulate polyacetal carboxylate produce a significant amount of very small, dust-like particles which settle to the bottom of a box of detergent. Removal of the dust-like particles by sieving prior to addition of the particulate polyacetal carboxylate mixture to the detergent mixture results either in excessive waste of polyacetal carboxylate, or in an expensive recycle process involving re-wetting and drying of the recycled dust-like particles to produce larger particles. Dynamic drying sometimes also results in the formation of undesirably large particles. Polyacetal carboxylate particles, therefore, commonly separate from detergent particles in the detergent box, resulting in a substantially non-uniform dispersion of polyacetal carboxylate throughout the detergent box.
Further, centrifuging of polyacetal carboxylate precipitated from the slurry is a very expensive operation, requiring excessive maintenance cost for operating equipment. It is also difficult to seal the centrifuge from air permeation. Since the centrifuged mixture contains methanol, the presence of air permitted by leakage through an ineffective seal creates a risk of explosion. Moreover, the standard drying steps entail introduction of heat to induce drying within reasonable times. Application of heat not only adds to energy costs, but involves still further expense because excessive care is required in handling the washed, centrifuged precipitate which contains highly flammable residual methanol.
In addition, polyacetal carboxylate has been found to be very temperature sensitive. Accordingly, addition of heat to induce drying often discolors the product, turning it yellow or brown. It is preferable from an aesthetic standpoint that the polyacetal carboxylate retain its white color so that a visually appealing product can be incorporated directly into detergent. Moreover, if a particular color of builder is desired, this is most readily and reliably achieved by dying a white polyacetal carboxylate. Yellow or brown color bodies in the product may interfere with the color of the dye.
Moreover, in view of the time constraints imposed by the cleaning cycles of, for example, clothes washers and dish washers, it is also desirable that the polyacetal carboxylate composition dissolve in water relatively rapidly.